The sound from a loudspeaker can be reflected or coupled back to a microphone after some finite delay, producing an echo. Many modern audio devices include circuits known as acoustic echo cancelers, for reducing or eliminating the effects of such echoes. In an ideal case, the echo corresponds to the electrical signal causing the loudspeaker to generate the sound and the audio device (including loudspeaker, enclosure, and microphone) may be assumed to have a linear response to such electrical signal. However, in reality, most audio transducers, such as microphones and loudspeakers, and components used in devices including microphones and loudspeakers (e.g., power amplifiers used to drive loudspeakers) are nonlinear, even when operated in their optimum operating ranges.
These nonlinear effects may significantly reduce the performance of acoustic echo cancelers, particularly in mobile phones and hands-free kits for mobile phones, as such devices may often use inexpensive, low-quality loudspeakers that may be poorly isolated from the remainder of the audio device. When such a loudspeaker is overdriven, saturation effects associated with the loudspeaker and its amplifier distort sound in a nonlinear manner. An acoustic echo of such sound contains a mixture of linear and nonlinear components. A typical acoustic echo canceler estimates only the linear acoustic impulse response of the loudspeaker-enclosure-microphone system. The remaining nonlinear components can be large and audible, particularly at high volumes.
Shortcomings mentioned here are only representative and are included simply to highlight that a need exists for improved electrical components, particularly for echo cancellation employed in consumer-level devices, such as mobile phones. Embodiments described herein address certain shortcomings but not necessarily each and every one described here or known in the art.